1. Field of the Invention
The present invention relates to a process for preparing a carboxylic ester of an aromatic compound having a hydroxymethyl group on the aromatic ring by reacting an aromatic compound having a methyl group on the ring, an aliphatic carboxylic acid, and molecular oxygen.
2. Description of Prior Art
It has been known that noble metals of Group VIII of the Periodic Table, especially palladium, are effective as catalysts for the reaction described above (Japanese Patent Publication No. 13081/1967). It also has been known to add auxiliary catalysts, such as bismuth compounds, gold compounds, tin salts, organic phosphorus compounds or iron salts, etc. in order to enhance the catalytic activity. (Japanese Patent Application Publication No. 18843/1972 and U.S. Pat. No. 3,547,982). In the present case, a carboxylic acid salt of a metal of Groups IA or IIA of the Periodic Table can be added as an auxiliary activator. However, from the viewpoint of catalytic activity, some of these catalysts have been found to be unsatisfactory for industrial purposes. They suffer from disadvantages including the following. In order to increase the activity of palladium as a catalyst, high quality palladium has been employed, thereby substantially increasing cost. In the gaseous reactions, a decrease of the catalytic activity of the catalyst arises, in many instances within a short period of time. Moreover, in the conventional technology for this reaction, carboxylic esters of aromatic compounds having a hydroxyl group on the aromatic ring have been produced as a substantial side-product; in fact, so substantial that it often is the main product. Consequently, it would be desirable to have an inexpensive catalyst with a long lifetime which does not allow the formation of the aforementioned by-products.